1. Field of the Invention
The present invention relates to a focusing and tracking servo circuit used in an optical disk device, and, more particularly, to a focusing and tracking servo circuit that performs recording and reproduction on a rewritable optical disk.
2. Description of the Related Art
There are two types of recording optical disks: one is a writing once type, and the other is a rewritable type. A CD-R (Compact Disk Recordable), which is a writing once optical disk, and a CD-RW (Compact Disk Rewritable), which is a rewritable optical disk, are both provided with a pregroove for guiding. Such a pregroove is very slightly wobbled in the radial direction with a center frequency of 22.05 kHz. Address information called ATIP (Absolute TimeIn Pregroove) for recording is multiplexed by FSK (Frequency Shift Keying) modulation with a maximum shift of .+-.1 kHz, and recorded on the pregroove.
A focusing and tracking circuit used in an optical disk device that performs recording and reproduction on such a recording optical disk emits optical beam on the optical disk, detects the reflection light from the optical disk by a plurality of detectors, generates a focusing and tracking error signal by performing a predetermined arithmetic operation, and drives a focusing and tracking actuator based on the focusing and tracking error signal.
In an optical disk device that performs recording and reproduction on a CD-R, which is a writing once optical disk, light beam power is used as read power at a time of reproduction. At a time of recording, the light beam power is switched between write power and read power (write power&gt;read power) in accordance with the value 0 or 1 of a recording signal. Accordingly, at a time of recording as well as reproduction, the reflection light is sampled at the timing of the light beam power being switched to the read power, thereby generating a focusing and tracking error signal.
In an optical disk device that performs recording and reproduction on a CD-RW, which is a rewritable optical disk, the light beam power is switched between write power and erase power (write power&gt;erase power&gt;read power) at a time of recording, in accordance with the value 0 or 1 of the recording signal. Accordingly, at a time of reproduction, the reflection light is detected at the timing of the light beam power being switched to the read power, thereby generating the focusing and tracking error signal. On the other hand, at a time of recording, the reflection light is sampled at the timing of the light beam power being switched to the erase power, thereby generating the focusing and tracking error signal.
Since the erase power has a higher output than the read power, the servo gain varies with the power. Therefore, it is necessary to change the sensitivity of each detector and the servo gain for recording and reproduction.
FIG. 1 is a block diagram of a conventional detector sensitivity switching circuit. This circuit is disposed inside a head amplifier IC. As shown in FIG. 1, a detector that detects a main light beam spot is divided into four detectors 10A to 10D, a detector that detects a preceding sub beam spot is divided into two detectors 10E and 10F, and a detector that detects a following sub beam spot is divided into two detectors 10G and 10H.
The head amplifier IC performs serial data transfer in order to reduce the number of pins. In compliance with a recording command or a reproduction command that instruct to switch between recording and reproduction, data SDATA consisting of address data and parameter data for setting detector sensitivity is serially transferred from a host device, and then stored in a shift register 12 at the timing of a clock signal SCK. An address decoder 14 decodes the address data stored in the shift register 12, and transmits a write enable signal to one of detector sensitivity registers 16A to 16H corresponding to the address. The detector sensitivity register (16B, for instance) that has received the write enable signal stores the parameter data supplied from the shift register 12. Thus, the detector sensitivity of the detector (10B, for instance) corresponding to the detector sensitivity register (16B, for instance) can be switched.
Since the parameter data is serially transferred, a certain period of time is required until the detector sensitivities of all the detectors 10A to 10H are switched. FIG. 2 shows the switching timing of detector sensitivity in the prior art. Based on the ATIP information (timing information) of a wobble signal reproduced from the disk, a write gate is opened by a signal processing circuit, and recording is started. As shown in FIG. 2, the detector sensitivities of the detectors 10A to 10H are serially switched immediately before and after the opening of the write gate. As a result, there is a problem that the focusing servo and tracking servo become unstable.
This problem also arises in a case where the servo gain of the focusing and tracking servo is switched. While the servo gain is serially switched, the focusing servo and tracking servo become unstable. Furthermore, the offset varies with the switching of servo gain. If the switching of servo gain and the switching of offset are nor performed at the same time, the focusing servo and tracking servo become unstable.